The present invention is applicable, in particular, in connection with marine risers used in offshore oil extraction, although it has potential applications in other situations in which a submersed, elongate (and typically cylindrical) member is exposed to water flow and must be protected from the effects thereof. Therefore although risers will be referred to in the following discussion, the member in question may take other forms—it may for example be a pipe, drilling riser or indeed it may be a buoyancy unit, e.g. a unit attached to a marine user.
Water currents impinging on marine risers can create both vortex induced vibration and drag.
Vortices form in the flow downstream of the riser and are intermittently “shed”—that is, they detach themselves from the riser and move downstream with the flow. Vortices shed from one side or other of the riser create a force upon it in a corresponding direction, generally across the direction of fluid flow. It is found that in some cases this shedding of vortices can cause a low frequency vibration or oscillation of the riser. It is believed that while the shedding of vortices is incoherent along the length of the pipe, their net effect is relatively small. If vortices correlate along the pipe length to produce a coherent effect upon it then potentially damaging VIV can result. Motion of the pipe, for example periodic motion back and forth in a direction across the fluid flow, can promote vortex correlation and so exacerbate the problem. If VIV occurs at or near a natural frequency of which pipe oscillation; resonance effects can exacerbate the problem.
Drag is a force on the riser along the direction of the fluid flow. It produces undesirable bending stresses and potentially it also increases load at the riser's point of suspension.
It is known to address both problems by placing around the riser a streamlined, teardrop shaped fairing which is free to pivot about the riser's axis and so to “weathervane”—that is, to align itself with the direction of the current. Such a fairing is disclosed in the applicant's UK patent application 0603040.7 published under number GB 2421751 A. The fairing disclosed in that application is formed of multiple fairing sections placed side-by-side along the length of the riser. To locate the fairings and prevent them from moving along the riser, clamps are interposed at intervals between neighbouring fairings, the clamps being secured to the riser. Such fairing arrangements are found to be effective in suppressing VIV, although improvement of their performance remains desirable.
Another approach to suppression of VIV is to provide shaped features upon the riser which influence the transition, in the fluid flow over it, from smooth laminar flow to turbulent flow. A widely adopted arrangement of this type uses helical strakes extending along the riser's length and around its circumference. While effective in suppressing VIV, these strakes add constructional complexity, and can also increase drag as well as creating difficulties during deployment of the riser.
It has been observed in confidential trials that a fixed cylindrical pipe, exposed to a flow normal to its length, while shedding vortices as a result of the flow, is subjected to smaller oscillatory forces than an identical pipe supported in a manner that allows movement normal to the direction of flow.
This has been explained by reference to correlation of vortices along the length of the pipe. There is a “correlation length” beyond which—in the case of a fixed pipe—vortices do not correlate together, and so remain out of phase. Their effect on the pipe is therefore incoherent and any tendency toward VIV is thus reduced. Where the pipe is able to oscillate, its motion is thought to promote correlation, and the resultant coherent effect of the shed vortices further promotes VIV.
It has also been observed, in confidential trials conducted by the applicant, that in the type of arrangement described above comprising rotatable fairings separated by clamps, omission of the clamps from the arrangement can impair VIV suppression. It is believed that in this case vortices shed from the fairings along the length of the riser correlate, so that their action upon the riser is coherent. When the shedding frequency approaches the natural frequency of the riser, resonance can occur leading to deleterious effects. The presence of the clamps is believed to assist in reducing the correlation between vortices, so that their effect is less coherent and less likely to produce vibration.